1. Field of the Invention
The invention relates to a control apparatus for a four-wheel drive vehicle.
2. Description of Related Art
A type of four-wheel drive vehicle constantly transmits a driving force of an engine that is a driving source to main driving wheels that are first driving wheels of front and rear wheels, while transmitting, according to a traveling state, the driving force of the engine to auxiliary driving wheels that are second driving wheels of the front and rear wheels. An example of this type of four-wheel drive vehicle enables blocking of transmission of the driving force between the engine and a propeller shaft that transmits the driving force to the auxiliary driving wheels and between the propeller shaft and the auxiliary driving wheels, during two-wheel driving in which the driving force of the engine is transmitted only to the main driving wheels (see Japanese Patent Application Publication Nos. 2009-166706 (JP 2009-166706 A) and 2014-054880 (JP 2014-054880 A)). This type of four-wheel drive vehicle has recently been gathering attention because the vehicle allows rotation of the propeller shaft to be stopped during the two-wheel driving to suppress traveling resistance resulting from rotational resistance to the propeller shaft, enabling a reduction in fuel consumption.
The four-wheel drive vehicle described in JP 2009-166706 P automatically sets up a two-wheel driving state during traveling in a four-wheel driving state if a slip rate of the main driving wheels is equal to or lower than a predetermined two-wheel driving switching value, and automatically returns to the four-wheel driving state during traveling in the two-wheel driving state if the slip rate exceeds a predetermined four-wheel driving switching value.
The four-wheel drive vehicle described in JP 2014-054880 A has a center clutch between an output shaft of a transmission and the propeller shaft, and a pair of rear clutches between the pair of rear wheels serving as the auxiliary driving wheels and a rear differential apparatus. A control unit that controls the center clutch and the rear clutches releases the center clutch and the rear clutches to set up a two-wheel driving mode (FWD mode) in which the driving force is transmitted only to the front wheels when a road is estimated to be in a high friction state, and engages the center clutch and the rear clutches to set up a four-wheel driving mode (AWD mode) in which the driving force is transmitted to the front wheels and the rear wheels when the road is estimated to be in a low friction state.
The control unit engages the center clutch when the road is estimated to be in a medium friction state that is intermediate between the high friction state and the low friction state, while releasing the rear clutches to set up a standby mode in which the propeller shaft is rotated by a torque transmitted through the center clutch. In the standby mode, when need arises to quickly engage the rear clutches, vibration caused by the moment of rotary inertia of the propeller shaft and the like can be suppressed.
In the four-wheel drive vehicle described in JP 2009-166706 A, for example, when a quick acceleration in the two-wheel driving state rapidly increases the driving force transmitted to the main driving wheels, the slip rate of the main driving wheels becomes higher to cause the four-wheel driving state to be set up. However, the clutches are engaged after the increase in slip rate, leading to a temporal delay. Thus, a slip of the main driving wheels before setup of the four-wheel driving state may temporarily degrade traveling stability.
In the four-wheel drive vehicle described in JP 2014-054880 A, when the road is in the medium friction state, the propeller shaft rotates even in the two-wheel driving state, allowing a quick shift to the four-wheel driving state. However, for example, a situation where a quick shift from the two-wheel driving state to the four-wheel driving state needs to be achieved by performing a quick acceleration may occur even when the road is in the high friction state. In such a case, the temporal delay in the shift to the four-wheel driving state may rapidly increase the driving force transmitted to the main driving wheels, leading to a higher likelihood of slip of the main driving wheels.